The present invention generally relates to speech transmission systems, and more particularly to a compander using a logarithmic chain that permits flexibility in changing the compression and expansion characteristics.
In the transmission of speech, amplitude compression at the transmitting end with subsequent expansion at the receiving end is called companding, i.e., compressing-expanding. Companding can be performed on the analog signal before linear coding or with a nonlinear encoder. An example of the latter is described in Transmission Systems for Communications writted by Members of the Technical Staff of Bell Telephone Laboratories and published by Bell Telephone Laboratories, fourth revised edition, in 1971 at pages 574 to 583. Of course, the transmitted and received signals are not always coded, and in the simplest form of this technique, the amplitude compressed analog signal is transmitted and received.
In existing companders, the compression and expansion law is rigidly "built into" the compressor and expander. The compression or expansion characteristics can be determined by a variable-loss device. Examples of this approach are described by R. O. Carter, "Theory of Syllabic Companders," Proc. IEE, Vol. 111, No. 3, March 1964, pages 503 to 513, and D. Thomson, "A Speech Compander Using Junction Transistors," Post Office Elect. Engrs'., 1962, pages 14 to 18. In another approach, the compression and expansion characteristics can be determined by a fixed circuit configuration as described by H. G. Vandemark, "A Single-Channel-Per-Carrier Terminal for Satellite Communications," California Microwave Co. Publication, Dec. B 13, 1974, pages 8 to 10.
The principle reason for using companders in the transmission of speech is to improve the signal-to-noise ratio. Some factors contributing to the signal-to-noise ratio improvement are as follows. In the no speech periods, the expander attenuates the line noise (including cross-talk) picked up between the compressor and expander. The speech following silent periods is made more intelligible due to the listener's adaptation to the lower background noise. Moreover, the means speech power is increased in the channel compared to the uncompanded level. This is discussed in some detail by Eitel M. Rizzoni, "Compander Loading and Noise Improvement in Frequency Division Multiplex Radio-Relay Systems," Proc. IRE, February 1960, pages 208 to 220.
As described by D. G. Pape, "Modulation and Speech Processing Techniques for a Maritime-Satellite Service," IEEE Satellite Systems for Mobile Comm. and Surveillance, Conf. Publ. No. 95, 13-15 March 1973, pages 56 to 61, the British Post Office uses for the overall subjective audio SNR, (S.sub.A /N).sub.S, the following empirical formula: EQU (S.sub.A /N).sub.S =S.sub.A /N+5/6U-2/3N-1/6S
where
S.sub.A /N=unweighted mean audio signal-to-noise ratio (dB) PA1 U=unaffected level (dBmO) PA1 S=mean signal level (dBmO) during speech PA1 N=mean noise level (dBmO) PA1 n.sub.s and n.sub.s '=output levels PA1 n.sub.eo and n.sub.eo '=input levels (corresponding to 0 dBmO) PA1 n.sub.so and n.sub.so '=output levels corresponding to input levels n.sub.eo and n.sub.eo ', respectively.
In the compander action, the compressor on the transmitter side compresses the volume range of syllables with the constant ratio of .alpha. in dB with respect to the unaffected level. At the receiving side the volume range of syllables is expanded with the ratio of .beta.. The International Telegraph and Telephone Consultative Committee (CCITT) in the Blue Book, Vol. 3, Third Plenary Assembly (Geneva: May 25 to June 26, 1964), Recommendation G-162, page 62, recommends 0 dBmO for the unaffected level and .alpha.=.beta.=2. More specifically, Recommendation G-162 defines the compressor and expander characteristics for a telephony compander as follows: ##EQU1## where n.sub.e and n.sub.e '=input levels
By expressing n.sub.s and introducing voltage u.sub.o associated with levels n.sub.s and similarly voltage u.sub.i associated with input level n.sub.e, the preceding compression formula yields, ##EQU2## for compression and similarly EQU u.sub.o =K.sub.2 u.sub.i.sup..beta. =K.sub.2 u.sub.i U.sub.i.sup.(.beta.-1) ( 2b)
expansion, where U.sub.i is the rms (or pseudo-rms) value of u.sub.i averaged over the time of one syllable, K.sub.1 =u.sub.io (.alpha.-1/.alpha.), K.sub.2 =u.sub.io.sup.1-.beta., and u.sub.io denotes the "unaffected voltage level," which remains unchanged at the compression and at the expansion. With the unaffected level chosen as 0 dBm (0.775 V), Table 1 gives the numerical values for K.sub.1 and K.sub.2.
TABLE 1 ______________________________________ .alpha. = .beta. 2 3 4 ______________________________________ K.sub.1 0.88 0.84 0.83 K.sub.2 1.29 1.6 2.15 ______________________________________
The following are equivalent to equations (2a) and (2b): EQU Compression: u.sub.o =K.sub.3 u.sub.i U.sub.o.sup.(1-.alpha.) ( 3a) EQU Expansion: u.sub.o =K.sub.4 u.sub.i U.sub.o.sup.(.beta.-1/.beta.) ( 3b)
where EQU K.sub.3 =K.sub.1.sup..alpha. and K.sub.4 =K.sub.2.sup.2-.beta..